drm/amd/display: Added support for individual control for multiple back-light instances.

[Why & How]
Added support for individual control for multiple back-light instances.

Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>